How to Calculate N Loss After Torrential Rains

Exact predictions are difficult, but these guidelines will help you determine if you need supplemental N following heavy rainfalls.

Did you know that wet soils in June are much more conducive to nitrate loss, as compared to early spring? That’s because soils are warmer, and losses can mount as saturation and tile flow continues, according to John Sawyer, Extension soil fertility specialist with Iowa State University.

"One way to determine N loss is to calculate an estimate," he says. "Predicting the exact amount is difficult as many factors affect losses. However, estimates can provide guidance for supplemental N applications."

Estimating N loss is a two-step process, he says. Step one is to estimate how much ammonium has converted to nitrate-N. It’s a safe assumption that late-fall anhydrous ammonia and manure ammonium has by now nearly completely converted to nitrate. Preplant N applications will also have a majority to nitrate, although you should factor in whether you used a nitrification inhibitor. On the other hand, ammonium applications within the last two weeks will still be predominately in the ammonium form, especially for anhydrous ammonia.

Step two is to estimate how much nitrate-N has leached away. Some factors that can accelerate N loss include soil type (coarse-textured soil leaches N much more quickly than silt loam or clay loam soils), whether or not the field is tiled, and of course length of soil saturation. Research indicates an average 4% to 5% N loss per day that soils are saturated.

Here’s an example. Say you made a preplant application of UAN solution at 120 lbs per acre. Estimate that 95% of that application has since been converted to nitrate and soils were subsequently saturated for 10 days. The N loss estimate calculation looks like this:

Increase the loss estimate for tile-drained fields, or fields with sandier soils. In fact, on very coarse-textured/sandy soils, a rainfall of 4" to 6" on already moist soils could easily result in all nitrate leaching out of the crop rooting zone, Sawyer says.